In order to solve the problem of signal delay in semiconductor devices such as Large Scale Integration (LSI) devices used in information communication apparatuses, low-dielectric-constant films (so-called low-k films) are used as interlayer insulating films, and interconnection structures using copper (Cu), which has low electrical resistivity, as the material of interconnects and interconnect vias have been developed. In such an interconnection structure, it is common practice to cover interconnects with a barrier film containing a high-melting-point metal in order to prevent Cu atoms from diffusing into the interlayer insulating film.
Miniaturization of interconnection structures, among other factors, has aroused a demand for formation of a barrier film having a uniform thickness of several nanometers. However, it is difficult to uniformly form such a thin film by current Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) methods. Semiconductor devices having Cu interconnection structures in which barrier films are formed by these techniques may not provide a high degree of  reliability. Therefore, as an alternative to these methods, a technique has been proposed in which a Cu alloy is used as the interconnect material and an element in the alloy is caused to react with an element in an interlayer insulating film during a heating process to form a Cu diffusion barrier film.
[Patent Document 1]
Japanese Patent Laid-Open No. 2005-277390
In the method described in Patent Document 1, an alloy of Cu and another metal (added metal) is formed on the surface of an interconnect trench formed in an interlayer insulating film. Then, the added metal is diffused by heating to form a Cu diffusion barrier film. The metal added to Cu is a metal such as manganese (Mn) that has a high oxidation tendency and diffuses more readily than Cu. By using the method, a diffusion barrier film having a composition of MnSiXOY is formed at the interface between the Cu layer and the interlayer insulating film in a self-aligned manner.
However, if a Cu diffusion barrier layer is formed by the method disclosed in Patent Document 1, the concentration of an added metal such as Mn remaining in Cu interconnections needs to be low in order to keep the resistance of the Cu interconnections low.